Comments

but Fenrik has a "toward/away' shader with which you can have different shaders on either side of a polygon. I guess you could make one side transparent?
Put it in the alpha chanel and then invert.
Maybe not,http://www.daz3d.com/shop/pose-and-shading-tools-2

Also Inagoni has baker which will bake shadows onto a texture and let you export it - so the shadow will still be there even if object casting said shadow is not.

Maybe if you describe a bit more what you're trying to do...cause it's not real clear what result you want to get. You can remove objects from your render in post (using an "object index" pass), leaving only their shadows, but I'm guessing that might not be what you really want.

@PhilW: Of course! ...but with some compositing-work even that is possible.

ANYTHING is possible with some compositing work, but "fixing it in post" shouldn't be the goal. I'm still curious about the original intent. Something isn't quite adding up. I just hope he's not going roundabout for something he doesn't need to go roundabout on.

Maybe if you describe a bit more what you're trying to do...cause it's not real clear what result you want to get. You can remove objects from your render in post (using an "object index" pass), leaving only their shadows, but I'm guessing that might not be what you really want.

I agree, more information about the scene would help. I was thinking a shadow catcher as well, but they can also act as a mask. If there's an object in the scene that's behind it, or partially obscured by it, then it would be masked by it. Strangely, I used one once and mistakenly intersected the shadow catcher with a model (I was using it to mask the model I wanted it to appear later in an animation), and the model behind it was completely visible. Of course the alphas in the shader on the object may have confused Carrara as well.